HomeEnvironmentDitching diesel-only: greener alternatives to powering Britain's trains

Ditching diesel-only: greener alternatives to powering Britain’s trains

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Train travel is considerably greener than most modes of transport. Of the 126 million tonnes of greenhouse gas emissions attributed to the sector in 2017, railways were responsible for two million tonnes. Cars accounted for around 70 million and HGVs 21 million.

Nevertheless, change is required across society if the UK is to meet its legally binding commitment to reach net zero emissions by 2050.

Before that milestone agreement was reached, former rail minister Jo Johnson had already set the industry its own environmental challenge: phase out diesel-only trains by 2040.

Widespread electrification of the railway was “unlikely to be the only or most cost-effective way to secure … environmental benefits” he said, arguing that alternative forms of traction were the way forward.

So, what are the options and how have plans progressed? Stewart Thorpe reached out to David Shirres, editor of sister title Rail Engineer, to take a look at what forms of traction could power the country’s future passenger train fleets on non-electrified lines.

“I would like to see us take all diesel-only trains off the track by 2040. If that seems like an ambitious goal – it should be and I make no apology for that.

After all, we’re committed to ending sales of petrol and diesel cars by 2040. If we can achieve that, then why can’t the railway aspire to a similar objective?

Rail may be less carbon intensive than road transport. That’s why modal shift’s so important. Getting freight and passenger vehicles off the roads onto greener forms of transport. But that does not absolve the rail industry from cleaning up its own act.

You may have seen stories recently about transport becoming the most polluting sector of our economy. And the fact that rail emissions have actually increased in absolute terms. Up 33 per cent since 1990. This cannot go on.”

Jo Johnson, rail minister, February 2018

Bi and tri-mode

In 2017, soaring costs for electrification of the Great Western main line, the Midland main line and in the north led previous transport secretary Chris Grayling to curtail plans and focus on introducing “new bi-mode train technology”.

These trains are equipped with pantographs for electrified railways and diesel engines for non-electrified sections of the network.

“This means that we no longer need to electrify every line to achieve the same significant improvements to journeys,” said Grayling in July 2017.

A swell of orders has followed, most recently from East Midlands Railway for 33 bi-mode trains from Hitachi. 

Going one step further, Great Western Railway ordered 19 ‘tri-mode’ trains from Porterbrook in 2018. These trains are surplus Class 769 stock which have been converted to operate using overhead wires, third rail or diesel engines.

David said: “There is no justification for Grayling’s claim that bi-modes offer journey improvements that avoid the need for electrification. This is because, in diesel mode, they only have about two thirds of the power of their electric mode.

“Nevertheless, bi-modes do offer immediate decarbonisation benefits as they eliminate diesel running under electrified wires. For example, with the introduction of Azumas on the 523-mile London to Aberdeen route, only 131 miles of that journey will be diesel-powered, whereas previously diesel-powered HSTs operated this route throughout.

“The rail industry’s decarbonisation report makes it clear there is no role for bi-modes in a net zero carbon railway – in which all the routes on which they currently operate will be electrified. Bi-modes are, therefore, a transitional technology to decarbonisation.”


At the time of his announcement on electrification, Grayling also mentioned hydrogen as a potential alternative to diesel.While there are no hydrogen trains in service in the UK, there are two major projects to change that. 

Train manufacturer Alstom is working with leaser Eversholt Rail to create a UK variant of its hydrogen-powered Coradia iLint, which is in passenger use in Germany. Codenamed Breeze, this project involves the conversion of an existing Class 321 train for use on rural routes. The first Breeze could enter service as early as 2022.

Elsewhere, a rival collaboration between the Birmingham Centre for Railway Research and Education (BCRRE) and Porterbrook is also racing to create a hydrogen-powered train. Dubbed ‘HydroFlex’, the partnership is drawing on BCRRE’s experience building ‘Hydrogen Hero’, a scaled-down yet functional hydrogen-powered train. HydroFlex is based on a Class 319 train and is set for main line testing in the near future. 

David said: “The energy density of compressed hydrogen is such that the iLint train offers a range of about 1,000km at a performance between that of diesel and electric multiple-units. However, hydrogen is not suitable for high power applications such as high-speed, freight or commuter train services, which require high acceleration.

“Hydrogen can be produced by electrolysis using unwanted overnight wind power. Yet the production and use of hydrogen in this way is only a third of the efficiency of using electricity to power trains directly.

“On a future net-zero carbon railway, hydrogen trains will be required for rural services on lines where there will never be a case for electrification. Prior to then, there may be a requirement for them to replace DMUs on lines that are yet electrified.”


The final diesel alternative mooted by Grayling was to use batteries.

Two years before his announcement, a partnership of Network Rail, the Department for Transport, Bombardier, Greater Anglia and innovation body Future Railway successfully ran a battery-powered Class 379 train in passenger service for five weeks.

Known as the Independently Powered Electric Multiple-Unit (IPEMU), the train’s integrated battery system was charged from regenerative braking when running on electrified lines and at terminal stations. A pantograph was used on the overhead lines.

Network Rail described it as the first battery-powered train to run on Britain’s rail network in more than half a century and hoped the milestone demonstrated the feasibility of using batteries to power a train for short distances off the electrified network. Bombardier is currently conducting further testing with train battery technology in Austria and Germany.

Back in the UK, Vivarail is a pioneer in the field of modern train battery technology and trialled its battery train on the Bo’ness & Kinneil Railway in Scotland last year.

Hitachi has run a battery-only train – the Dencha – in southern Japan since 2016. A spokesperson said the company is working to secure its first order for battery trains for the UK.

David said: “Whilst battery technology is fast developing, in respect of energy storage it is unlikely ever to come close to diesel or exceed that of hydrogen. Costs are currently falling and this may well change further if and when all the engines in the world’s one billion cars need to be replaced by batteries.

“In the short-term, EMUs fitted with batteries could have a useful role in effectively extending electrification without wires for short distances on medium-speed passenger services. This will become progressively less useful if there is a rolling electrification programme which is also needed for freight, commuter and high-speed passenger services.

“Thus, in the long-term, there is likely to be limited use for battery-powered trains, which should be seen as a transitional technology.”


Chancellor of the Exchequer Gordon Brown launched the country’s first ‘biodiesel’ train in 2007. Virgin Trains’ Class 220 Voyager used a blended fuel that consisted of 20 per cent biodiesel –  derived from sources such as rapeseed, soyabean and palm oil – to reduce CO2 emissions.

Virgin Trains aimed to convert more of its fleet in the future as part of an attempt to make rail travel more environmentally friendly.

RSSB researchers concluded that a 20 per cent blend of biofuel mixed with 80 per cent diesel was the highest blend that could be universally accepted without significant expenditure to alter train engines. It also found that two key barriers to the take up of biodiesel were sustainability and the tax on the fuel source. At the time, the government heavily taxed biofuel, making wider adoption costly. The Treasury made an isolated concession for the trial, but beyond this the duty rate would have been 54.68p per litre compared to 7.69p per litre duty paid by the rail industry for diesel.

David said: “Growing plants to produce biofuels requires carbon, furthermore there is a practical limit to biofuel production as land is needed to grow food. For these reasons, it is unlikely that biodiesel will be available in sufficient quantities to rail to have anything other than a marginal impact.”


The abundance of natural gas in Russia led to the development of a gas turbine locomotive powered by liquefied natural gas (LNG) in 2007.

Although there has been no take-up in the UK rail industry, it is a developing area in Russia as a cheaper and lower emitting alternative to diesel.

David said: “LNG’s CO2 emissions are about 70 per cent of diesel. Although this offers a slight decarbonisation benefit, LNG needs to be stored at minus 162 °C. It is therefore unlikely to be part of any rail decarbonisation programme.”

Final thoughts 

David concluded: “Of all transport modes, rail is the most carbon friendly and the most straightforward to decarbonise. For these reasons, perhaps rail’s greatest contribution to the reduction of UK carbon emissions is by encouraging modal shift from planes and cars. However, to do this, the rail network needs the extra capacity that projects such as HS2 will provide. Rail also needs to offer an attractive alternative and so requires electric trains which offer high-speed and high acceleration. Therefore, in the long-term, electrification is needed for all except rural routes, which can be operated by hydrogen trains. There is unlikely to be any long-term role for bi or tri-mode, battery, biodiesel or LNG trains.

“Currently, the average electric passenger vehicle and freight locomotive have, respectively, 26 per cent and 14 per cent the carbon emissions of diesel trains. As electricity generation gets greener, these figures are expected to be 11 per cent and six per cent by 2040.

“Despite these benefits, there are concerns about the cost of electrification as schemes such as Great Western (GW) have been subject to unacceptable time and cost overruns. However, since then other electrification programmes have run to time and costs have fallen to 33-50 per cent of the GW scheme, as explained by the Railway Industry Association’s ‘Electrification Cost Challenge’ report. This also describes how new technology can further reduce costs by, for example, avoiding the need for bridge reconstruction.

“Electrification is a specialist activity and so is most efficiently done in a rolling programme in which competent teams can maintain and develop their skills. A rolling electrification programme of about 200 route km a year would electrify all but rural routes with infrequent services by 2040. This would result in about 95 per cent of passenger traffic and 75 per cent freight traffic being electric powered.  

“Hydrogen trains could then provide rural passenger services while some diesel locomotives would still be required for freight and engineering trains on unelectrified lines for which there would need to be carbon offsets. 

“Until then, the industry needs to develop a transitional strategy to develop the rolling electrification programme which makes the best use of bi/tri-modes, battery and hydrogen trains.”